A large sized liquid crystal display (LCD) have recently attracted attention. With respect to large-sized LCDs, required therein is higher performance, for example, in viewing angle, contrast and stability in quality against environmental changes, compared to the required performance for conventional personal notebook computers or LCD monitors. Therefore, higher performance is also required for the polarizing plates used for the large sized LCD device or for the cellulose ester film used for the polarizing plate. Specifically, with respect to a stretched cellulose ester film, the performance of which has a large effect on the optical property of an optical phase retardation film (hereinafter, merely referred to as a retardation film), more severe specifications are required as one of the important members of a LCD device. A cellulose film is provided with an optical phase retardation property by stretching the film. Using this method, it is not fully easy to obtain a stable retardation value by stretching when a cellulose triacetate film is used, which has been widely used for such film. In order to obtain a stable retardation value in a cellulose triacetate film, proposed have been methods to use additives, to select substituents or to control the acetylation degree of a cellulose triacetate film. However, the method to provide the desired retardation effect by stretching tends to fracture the film, and markedly lowers the productivity of the retardation film. Further, since retardation films of a higher stretching ratio have been produced to obtain the desired retardation value, a resulting problem has been a tendency to cause haze.
In order to improve the display performance of an LCD, a backlight unit has more commonly been provided behind the liquid crystal cell to directly illuminate the liquid crystal cell without usin a light guide. However, in this direct illumination backlight system, when a LED backlight unit is used instead of a fluorescence tube backlight unit, deterioration of display quality due to leakage of light in the peripheral areas of the display, also known as “corner spots” tends to occur in accordance with the temperature increase caused by heat generated from the LED backlight unit, and further improvement to overcome the drawback has been desired.
For example, in Patent Document 1, an optical film employing cellulose acetate propionate is proposed. The aim of this optical film is to attain low optical anisotropy and high heat resistance, but not to effectively utilize the optical anisotropy. Plasticizers, for example, a phosphate ester, a fatty acid ester, a phthalate ester and a citrate ester, disclosed in this patent document are compounds which reduce the optical anisotropy rather than increase the optical anisotropy.
In Patent Document 2, a cellulose ester film containing a citrate ester plasticizer is disclosed. However, this film also aims to obtain a cellulose ester film exhibiting small optical anisotropy. It has been thought to be difficult to use a citrate ester plasticizer for the purpose of effectively utilizing the optical anisotropy.
In Patent Document 3, a method to prepare a retardation film is disclosed. When a cellulose ester film is stretched to provide a retardation effect to the cellulose ester film, both in-plane retardation value and out-of-plane (in the thickness direction) retardation value change by stretching, and the control of these retardation values are not fully easy. Specifically, when a citrate ester is mixed with the cellulose ester, since it has an effect to reduce anisotropy of the film, it is not fully easy to keep the Rt/Ro value at not less than 1. Also, improvement in an environmental variation of a retardation value and light leakage is desirable.
In order to obtain a display exhibiting long term stability of display quality and high productivity, disclosed is a retardation film of which moisture permeability is reduced (for, example, refer to Patent Document 4). Also, in order to obtain a display exhibiting limited light leakage caused by thermal distortion and high display quality, disclosed is a retardation film of which expansion coefficient due to moisture absorption is suppressed below a prescribed value (for example, refer to Patent Document 5). A polarizing plate exhibiting high durability even under a high humidity-high temperature condition is also disclosed, which is attained by controlling the diffusion coefficient of boric acid in the polarizing plate (for example, refer to Patent Document 6). In this patent document, disclosed is a method to control the diffusion coefficient of boric acid by decreasing free volume in a cellulose acylate film, and in paragraph [0015] of this patent document, it is described that the free volume in a cellulose acylate film can be decreased by increasing the amount of crystals in the film. However, with respect to this polarizing plate, improvement in retardation value stability and dimensional stability under severe conditions have further been desired. Specifically, in an amorphous-like cellulose ester, for example, a mixed acid ester of cellulose such as cellulose acetate propionate, a notable effect has not been fully obtained.
(Patent Document 1) Japanese Patent Publication Open to Public Inspection (hereafter referred to as JP-A) No. 9-90101
(Patent Document 2) JP-A No. 11-92574
(Patent Document 3) JP-A No. 2002-62430
(Patent Document 4) JP-A No. 2002-14230
(Patent Document 5) JP-A No. 2002-71955
(Patent Document 6.) JP-A No. 2004-279931